Clarify recovery from ffwd on delegated targets

Akin to the recovery from fast-forward attacks on the top-level
targets role, if a delegated targets role has been compromised,
the previously trusted delegated targets metadata and the
previously trusted snapshot metadata must be deleted.

This must happen so that the rollback attack check (*), which makes
sure that the version number of the new delegated targets is higher
(or equal) than that of the old does not prevent updates after an
ffwd attack.

For the top-level targets metadata ffwd recovery logic is performed
based on key removals in the root metadata and thus can happen
before downloading the snapshot metadata.
For delegated targets, on the other hand, where the keys are defined by
delegating targets role(s) and not in the root metadata, ffwd
recovery logic can only be performed after the delegating targets
have been downloaded.

(*) Note that there are two targets role rollback checks. One is
based on the snapshot metadata, to fail early, i.e. before a
potentially compromised (delegated) targets metadata is downloaded,
and the other is based directly on the (delegated) targets
metadata, so that an attacker needs to compromise snapshot and
(delegated) targets keys, to successfully perform a rollback
attack.

This commit updates the client workflow according to above
observations.

Author: lukpueh

tuf-spec.md

@@ -1220,14 +1220,13 @@ non-volatile storage as FILENAME.EXT.
     metadata file, discard it, abort the update cycle, and report the potential
     rollback attack.
 
-    * **3.3.2**. The version number of the targets metadata file, and all
-    delegated targets metadata files (if any), in the trusted snapshot metadata
-    file, if any, MUST be less than or equal to its version number in the new
-    snapshot metadata file. Furthermore, any targets metadata filename that was
-    listed in the trusted snapshot metadata file, if any, MUST continue to be
-    listed in the new snapshot metadata file.  If any of these conditions are
-    not met, discard the new snapshot metadadata file, abort the update cycle,
-    and report the failure.
+    * **3.3.2**. The version number of the top-level targets metadata file, in
+    the trusted snapshot metadata file, if any, MUST be less than or equal to
+    its version number in the new snapshot metadata file. Furthermore, any
+    targets metadata filename that was listed in the trusted snapshot metadata
+    file, if any, MUST continue to be listed in the new snapshot metadata file.
+    If any of these conditions are not met, discard the new snapshot metadadata
+    file, abort the update cycle, and report the failure.
 
   * **3.4**. **Check for a freeze attack.** The latest known time should be
   lower than the expiration timestamp in the new snapshot metadata file.  If
@@ -1287,7 +1286,18 @@ non-volatile storage as FILENAME.EXT.
       * **4.5.2.1**. Let DELEGATE denote the current target role TARGETS is
       delegating to.
 
-      * **4.5.2.2**. **Download the DELEGATE targets metadata file**, up to either
+      * **4.5.2.2**. **Fast-forward attack recovery.** If a threshold of
+      delegated targets keys for the current delegation are removed from the
+      TARGETS metadata, delete the trusted DELEGATE metadata, if any, and the
+      previously trusted snapshot metadata.
+
+      * **4.5.2.3**. **Check for a rollback attack via snapshot.** The version number of the
+      DELEGATE metadata in the previous trusted snapshot metadata, if any, MUST
+      be less than or equal to its version number in the new trusted snapshot
+      metadata. If this is not the case, abort the update cycle, and report the
+      potential rollback attack.
+
+      * **4.5.2.4**. **Download the DELEGATE targets metadata file**, up to either
       the number of bytes specified in the snapshot metadata file, or some Z
       number of bytes. The value for Z is set by the authors of the application
       using TUF. For example, Z may be tens of kilobytes. IF DELEGATE cannot be
@@ -1300,39 +1310,40 @@ non-volatile storage as FILENAME.EXT.
       in the snapshot metadata file.  In either case, the client MUST write the
       file to non-volatile storage as FILENAME.EXT.
 
-      * **4.5.2.3**. **Check against snapshot metadata.** The hashes (if any), and
+      * **4.5.2.5**. **Check against snapshot metadata.** The hashes (if any), and
       version number of the new DELEGATE metadata file MUST match the trusted
-      snapshot metadata.  This is done, in part, to prevent a mix-and-match
+      snapshot metadata, if any.  This is done, in part, to prevent a mix-and-match
       attack by man-in-the-middle attackers. If the new DELEGATE metadata file
       does not match, abort the update cycle, and report the failure.
 
-      * **4.5.2.4**. **Check for an arbitrary software attack.** The new DELEGATE
+      * **4.5.2.6**. **Check for an arbitrary software attack.** The new DELEGATE
       metadata file MUST have been signed by a threshold of keys specified in the
       TARGETS metadata file.  If the new DELEGATE metadata file is not signed
       as required, abort the update cycle, and report the failure.
 
-      * **4.5.2.5**. **Check for a rollback attack.** The version number of the
-      trusted DELEGATE metadata file, if any, MUST be less than or equal to the
-      version number of the new DELEGATE metadata file.  If the new DELEGATE
-      metadata file is older than the trusted DELEGATE metadata file, discard
-      it, abort the update cycle, and report the potential rollback attack.
+      * **4.5.2.7**. **Check for a rollback attack on the DELEGATE metadata.**
+      The version number of the trusted DELEGATE metadata file, if any, MUST be
+      less than or equal to the version number of the new DELEGATE metadata
+      file.  If the new DELEGATE metadata file is older than the trusted
+      DELEGATE metadata file abort the update cycle, and report the potential
+      rollback attack.
 
-      * **4.5.2.6**. **Check for a freeze attack.** The latest known time
+      * **4.5.2.8**. **Check for a freeze attack.** The latest known time
       should be lower than the expiration timestamp in the new DELEGATE
       metadata file. If so, the new DELEGATE file becomes the trusted DELEGATE
       file. If the new DELEGATE metadata file is expired, abort the update
       cycle, and report the potential freeze attack.
 
-      * **4.5.2.7**. If the current delegation is a multi-role delegation,
+      * **4.5.2.9**. If the current delegation is a multi-role delegation,
       recursively visit each role, and check that each has signed exactly the
       same non-custom metadata (i.e., length and hashes) about the target (or
       the lack of any such metadata). Otherwise, abort the update cycle, and
       report the failure.
 
-      * **4.5.2.8**. If the current delegation is a terminating delegation,
+      * **4.5.2.10**. If the current delegation is a terminating delegation,
       then jump to step 5.
 
-      * **4.5.2.9**. Otherwise, if the current delegation is a non-terminating
+      * **4.5.2.11**. Otherwise, if the current delegation is a non-terminating
       delegation, continue processing the next delegation, if any, by repeating
       step 4.5 with DELEGATE as the current TARGET role. Stop the search, and
       jump to step 5 as soon as a delegation returns a result.